1. Field of the Invention
The present invention relates to UHF-band RFID tags, and concerns an RFID tag in which, even when a card-type 13 MHz band RFID tag is included with it, the effects on the loop antenna of the 13 MHz band RFID card are reduced, and good radiation and receiving characteristics can be obtained even when used in proximity to the human body.
2. Description of the Related Art
The RFID tag is explained using a conventional folded dipole antenna.
FIG. 1 shows the upper face of a thin RFID tag 500. A chip 510 placed on an aperture 550 is connected to contacts 525 on a flexible substrate 520, and the substrate 520 is has at least two folded dipole antennas 530 and 531 contained therein.
A wireless tag having a conventional ground plane is next explained.
FIG. 2 shows the configuration of a wireless tag 1. FIG. 2(a) is a plan view of the wireless tag 1 and (b) is a sectional view thereof.
The wireless tag 1 has a circular polarized wave matching form and is composed of an emitting-side conductive sheet 2 having a radio wave emitting face 2a, an earth-side conductive sheet 3, which has an earth face 3a, a semiconductor module 4 and a dielectric part 5. Here, an antenna element 6 is constituted by interposing the dielectric part 5 between the emitting-side conductive sheet 2 and earth-side conductive sheet 3, and the emitting-side conductive sheet 2 is one having a circular form with a rectangular cut-out area 7 (aperture) in its center. One terminal 4a of the semiconductor module 4 is connected to the emitting-side conductive sheet 2 and the other terminal 4b to the earth-side conductive sheet 3. The impedance observed between one point of the radio wave emitting face 2a from the dielectric part 5 in the earth face 3a, as is well-known, approaches 0 approaching the center of the radio wave emitting face 2a, and increases approaching the edge of the radio wave emitting face 2a. 
At the edge of the radio wave emitting face 2a, the impedance reaches a high level of several hundred ohms.
With respect to the position of the semiconductor module 4, the module is connected in a vicinity where the impedance observed from both terminals 4a and 4b of the semiconductor module 4 can be better adjusted to the impedance between the radio wave emitting face 2a and the earth face 3a. Adjustment is performed so as to include impedance characteristics dependent upon the length and width of the terminals 4a and 4b. The circularly-polarized signal emitted from the interrogator is captured by the radio wave emitting face 2a and input to the semiconductor module 4. When tag information is read, the signal wave that has been input to the wireless tag 1 is modulated according to the information in the wireless tag 1 and, by changing the tag impedance, the input wave is reflected and returned from the radio wave emitting face 2a to the interrogator.
Devices such as these are shown, for example, in Japanese Unexamined Patent Application Publication H8-88586 and Japanese Unexamined Patent Application Publication 2002-353735.
A card-type 13 MHz band RFID tag uses a coiled loop antenna and, when a separate UHF band or 2.45 GHz band RFID tag is superposed so as to cover the front or back face of this coiled loop antenna, the magnetic flux passing through the interior of the coiled loop antenna is blocked by the metal of the superposed UHF band or 2.45 GHz band RFID tag, and hardly any current is produced on the loop antenna. As a result, current is not supplied to the chip on the card-type 13 MHz band RFID tag, which fails to operate, and communication cannot be achieved.
As a separate problem from that described above, when a conventional UHF band RFID tag (e.g., a tag having a folded dipole antenna) is used in proximity to the human body, there has been the problem that the characteristics have deteriorated.
Moreover, while conventional tags having a ground plane may not suffer deterioration of characteristics in proximity to the human body, since the antenna has both a front and back side, characteristics have been known to deteriorate when the antenna element is turned toward the human body.